This invention pertains to protective surface coatings commonly known as paint coatings. Protective surface coatings are organic compositions applied to substrates to form continuous films which are cured or otherwise hardened to provide protection as well as a decorative appearance to the substrate. Protective surface coatings ordinarily comprise an organic polymeric binder, pigments, inert fillers and other additives. The polymeric binder functions as an organic vehicle for the pigments, inerts, and other additives in wet coating compositions and further functions as a binder for the pigments and inert fillers in the cured or hardened paint film. The polymeric binders of this invention can be thermosetting binders based on coreactive crosslinking components, if desired.
Polyurethane prepolymers for polyurethane dispersions are typically made in a solvent, such as acetone, at a low molecular weight with terminal isocyanate functionality. Acid functionality from compounds like dimethylol propionic acid renders these urethane prepolymers water dispersible after the addition of a tertiary amine. Some amount of an inverting solvent such as butyl cellosolve ordinarily is included. Once dispersed in water, a diamine (hexamethylene diamine) can be added to convert the remaining isocyanate groups to urea linkages and thus form a high molecular weight polyurethane urea. Solvent can then be removed by distillation. A disadvantage of this procedure is the use of organic solvent in producing the polyurethane prepolymer where the solvent has to be either removed by distillation or left in the polyurethane dispersion. Solvent not removed is retained in the finished paint which is detrimental to the cured coating film properties. Waterborne polyurethanes are disclosed in the Journal of Coated Fabrics. Vol. 16, pgs. 39-79 (Jul., 1986). Prior art patents disclosing water-dispersed polyurethanes include U.S. Pat. Nos. 4,335,029; 3,826,769; 4,305,858; 4,306,998; 4,147,680; 4,211,683; and 4,408,008.
Blends of a polyurethane with an acrylic polymer have a better balance of properties than either of the two polymers individually when evaluated alone. The polyurethane dispersion can be prepared as above, whereupon some or all of the organic solvent is removed by distillation, followed by the acrylic monomer and initiator being added to the preformed dispersion. The ethylenic monomers dissolve in the polyurethane particles, polymerize, and form a polymeric blend. However, this procedure still utilizes organic solvent in the preparation of polyurethane and some solvent can be retained in the composition.
It now has been found that aqueous dispersions of polyurethane/acrylic polymers can be prepared without organic solvent and, if desired, can be crosslinked at ambient or low bake temperatures with carbodiimide crosslinkers. Such compositions are particularly suitable for application and adhesion to temperature sensitive plastic substrates, where the cured paint films, particularly crosslinked films, exhibit excellent flexibility, adhesion, and water resistance when applied to several types of plastic substrates. Carboxyl functional latex resins crosslinked by polycarbodiimides are disclosed in European Patent Application Publication No. 0,121,083 published Oct. 10, 1984 based on Application No. 84101921.9 filed Feb. 23, 1984.
In accordance with this invention, a process has been discovered to prepare polyurethane/acrylic dispersions which does not require solvent during the synthesis of the urethane prepolymer. Materials for making the urethane prepolymer (diisocyanate, diol, dimethylolpropionic acid, catalyst, and optionally hydroxyl functional UV stabilizers, etc.) are first dissolved in ethylenically unsaturated monomers which serve as a solvent for synthesizing a low molecular weight urethane prepolymer as well as reducing the viscosity of the prepolymer to a processable level. Tertiary amine is added to invert this organic phase into a water dispersed organic phase of urethane prepolymer. Thereafter, the prepolymer is coreacted with primary or secondary amine to extend the isocyanate functional prepolymer into a high molecular weight polyurethane urea containing the monomers and dispersed in water. Thereafter the ethylenic monomers are copolymerized to provide a polyurethane-urea and acrylic copolymer dispersion in water. An improved product results and the process advantageously avoids the use and subsequent removal of organic solvent. The in-situ formed polymeric mixture of polyurethane-urea and acrylic copolymer stably dispersed in water can be used to provide a coalescent cured film or alternatively coreacted with a carbodiimide to provide a crosslinked cured film. These and other advantages of this invention will become more apparent by referring to the detailed description and illustrative examples.